Etched bismuth heart.
As you can see from several of the other samples on this site, bismuth makes lovely square hopper crystals. What you may not realize is that pretty much any plain chunk of bismuth is full of these crystals, whether you see them or not. The spectacular display crystals are made by allowing a pot of bismuth to partially harden, then removing the crystal before the liquid metal around it has a chance to solidify. But the crystal is still there even if you let the pot harden all the way, you just can't see it because it's completely surrounded by more solid bismuth.

The surface of a cooled lump of bismuth (and most other metals) is formed by the interrupted faces of different crystal zones, oriented in different directions relative to the surface. Normally the surface looks uniform, because the orientation of the underlying crystal zone doesn't have any effect on how light is reflected, probably because right at the very surface the atoms are disordered by their interaction with air at the moment of cooling.

However, if you use acid to etch away the outer layers of metal, you can expose the crystal zones in a way which makes them reflect light differently from different angles. The result is that you see a patchwork of zones that sparkle alternately depending on the angle of light. These flat zones in fact go way down into the solid, you're just seeing the ends of them.

The size of the zones depends on the purity of the metal (higher purity gives larger zones, and making big display crystals requires very high purity bismuth, at least 99.99%), and on the rate of cooling (slower cooling gives larger crystals a chance to form). Look at the rotation video for this sample and you'll see the crystal zones sparkling in a most lovely way.

Ethan has this to say about the process of etching he used:

I discovered that the best way to reveal the crystal interior after I cast the heart was an acid process, three seconds in industrial hydrofluoric acid and then seven in concentrated nitric. Unfortunately, in my zeal of preparing the baths and baking soda rinse, I spilled a drop or two of the hydrofluoric acid on my arm, and promptly washed it off for 15 seconds. I then proceeded to boy scouts for an hour or so, came back and started dinner. By that point the dime-sized patch on my arm was bright red and beginning to hurt like a jellyfish sting, so I told my parents what happened. Half an hour and one poison control call later, I was lying in a hospital bed with a big compress of 2% calcium gluconate around my arm, and a lot of shocked nurses, unfortunately at about midnight on a school night. Everything turned out fine, my arm stopped hurting, my heart didn't stop, I didn't acquire hypocalcemia, and the bismuth turned out beautifully!

From this we learn two things. First, that hydrofluoric acid is not to be trifled with (and neither is elemental fluorine for that matter). Second, that Ethan cooks.